Repairing the central nervous system is one of the frontiers that medical science has yet to conquer. Conditions such as Alzheimer's disease, Parkinson's disease, epilepsy, Huntington's disease, and stroke can have devastating consequences for those who are afflicted. Traumatic injury to the head or the spinal cord can instantly propel someone from the bounds of everyday life into the ranks of the disabled.
What makes afflictions of the nervous system so difficult to manage is the irreversibility of the damage often sustained. A central hope for these conditions is to develop cell populations that can reconstitute the neural network, and bring the functions of the nervous system back in line.
For this reason, there is a great deal of evolving interest in neural progenitor cells. Up until the present time, it was generally thought that multipotent neural progenitor cells commit early in the differentiation pathway to either neural restricted cells or glial restricted cells. These in turn are thought to give rise to mature neurons, or to mature astrocytes and oligodendrocytes. Multipotent neural progenitor cells in the neural crest also differentiate to neurons, smooth muscle, and Schwann cells. It is hypothesized that various lineage-restricted precursor cells renew themselves and reside in selected sites of the central nervous system, such as the spinal cord. Cell lineage in the developing neural tube has been reviewed in the research literature by Kalyani et al. (Biochem. Cell Biol. 6:1051,1998).
Putative multipotent neuroepithelial cells (NEP cells) have been identified in the developing spinal cord. Kalyani et al. (Dev. Biol. 186:202, 1997) reported NEP cells in the rat. Mujtaba et al. (Dev. Biol. 214:113, 1999) reported NEP cells in the mouse. Differentiation of NEP cells is thought to result in formation of restricted precursor cells having characteristic surface markers.
Putative neural restricted precursors (NRP) were characterized by Mayer-Proschel et al. (Neuron 19:773, 1997). These cells express cell-surface PS-NCAM, a polysialylated isoform of the neural cell adhesion molecule. They reportedly have the capacity to generate various types of neurons, but do not form glial cells.
Putative glial restricted precursors (GRPs) were identified by Rao et al. (Dev. Biol. 188:48, 1997). These cells apparently have the capacity to form glial cells but not neurons.
Ling et al. (Exp. Neurol. 149:411, 1998) isolated progenitor cells from the germinal region of rat fetal mesencephalon. The cells were grown in EGF, and plated on poly-lysine coated plates, whereupon they formed neurons and glia, with occasional tyrosine hydroxylase positive (putative dopaminergic) cells, enhanced by including IL-1, IL-11, LIF, and GDNF in the culture medium.
Wagner et al. (Nature Biotechnol. 17:653, 1999) reported cells with a ventral mesencephalic dopaminergic phenotype induced from an immortalized multipotent neural stem cell line. The cells were transfected with a Nurr1 expression vector, and then cocultured with VM type 1 astrocytes. Over 80% of the cells obtained were claimed to have a phenotype resembling endogenous dopaminergic neurons.
Mujtaba et al. (supra) reported isolation of NRP and GRP cells from mouse embryonic stem (mES) cells. The NRPs were PS-NCAM immunoreactive, underwent self-renewal in defined medium, and differentiated into multiple neuronal phenotypes. They apparently did not form glial cells. The GRPs were A2B5-immunoreactive, and reportedly differentiated into astrocytes and oligodendrocytes, but not neurons.
A number of recent discoveries have raised expectations that embryonic cells may become a pluripotential source for cells and tissues useful in human therapy. Pluripotent cells are believed to have the capacity to differentiate into essentially all types of cells in the body (R. A. Pedersen, Scientif. Am. 280(4):68, 1999). Early work on embryonic stem cells was done using inbred mouse strains as a model (reviewed in Robertson, Meth. Cell Biol. 75:173, 1997; and Pedersen, Reprod. Fertil. Dev. 6:543, 1994).
Compared with mouse ES cells, monkey and human pluripotent cells respond quite differently to culture conditions, and require different factors to be propagated in an undifferentiated state. Only recently have discoveries been made that allow primate embryonic cells to be cultured ex vivo.
Thomson et al. (Proc. Natl. Acad. Sci. USA 92:7844, 1995) were the first to successfully culture embryonic stem cells from primates, using rhesus monkeys and marmosets as a model. They subsequently derived human embryonic stem (hES) cell lines from human blastocysts (Science 282:114, 1998). Gearhart and coworkers derived human embryonic germ (hEG) cell lines from fetal gonadal tissue (Shambloft et al., Proc. Natl. Acad. Sci. USA 95:13726, 1998). Both hES and hEG cells have the long-sought characteristics of human pluripotent stem (hPS) cells: they are capable of ongoing proliferation in vitro without differentiating, they retain a normal karyotype, and they retain the capacity to differentiate to produce all adult cell types.
Reubinoff et al. (Nature Biotechnol. 18:399, 2000) reported somatic differentiation of human blastocysts. The cells differentiated spontaneously in culture, with no consistent pattern of structural organization. After culturing for 4-7 weeks to high density, multicellular aggregates formed above the plane of the monolayer. Different cells in the culture expressed a number of different phenotypes, including expression of β-actin, desmin, and NCAM.
Spontaneous differentiation of pluripotent stem cells in culture or in teratomas generates cell populations with a highly heterogeneous mixture of phenotypes, representing a spectrum of different cell lineages. For most therapeutic purposes, it is desirable for differentiated cells to be relatively uniform—both in terms of the phenotypes they express, and the types of progeny they can generate.
There is a pressing need for technology to generate more homogeneous differentiated cell populations from pluripotent cells of human origin.